Filter banks have emerged as useful tools in video signal processing. In particular, filter banks can be utilized to create a hierarchy of video sequences from a single high definition sequence, such that the individual channels in the hierarchy are compatible with various classes of existing television receivers. (See, W. F. Schreiber et al, "Channel Compatible 6-Mhz HDTV Distribution Systems," ATRP Tech. Rep. No. 79, MIT, Jan., 1988.)
Filter banks are central to subband coding techniques which have been used successfully in HDTV compression. (See, D. J. LeGall, H. Gaggioni and C. T. Chen, "Transmission of HDTV Signals Under 140 Mbit/s Using a Subband Decomposition and Discrete Cosine Transform Coding," in "Signal Processing of HDTV", pp. 287-293, L. Chiariglione, ed., North-Holland, 1988; R. Ansari, H. Gaggioni and D. LeGall, "HDTV Coding Using a Non-rectangular Subband Decomposition," SPIE Conf. on Visual Com. and Image Proc., Nov., 1988, Cambridge, Vol. 1001, pp. 821-824.) In a subband system, an analysis filter bank is used at a transmitter to decompose an HDTV sequence into a plurality of subband sequences, each of which is individually coded and transmitted. At a receiver, the individual subband sequences are decoded and recombined using a synthesis filter bank to reconstruct the image. Typically, in such a system, an HDTV sequence is first divided into low and high frequency subbands in the horizontal direction, and each of these subbands is then divided into low and high frequency subbands in the vertical direction, i.e., the analysis filter bank is "separable". While this type of scheme is highly advantageous for the compression of an HDTV video sequence, it does not result in a hierarchy of sequences which can be utilized by different classes of receivers.
A progressive video sequence is one in which each vertical-horizontal frame includes samples on all lines, while in an interlaced video sequence, in each frame there are samples on alternate lines. (Such frames wherein there are samples only on alternate lines are called fields.)
One non-separable filtering scheme of interest for television is quincunx subsampling over the vertical-time plane, since this allows one to go from a progressive sequence to an interlaced sequence and from an interlaced sequence to a progressive sequence. In quincunx subsampling every other sample is removed from each line, so that, for example, in the even numbered lines the even numbered samples are removed and in the odd numbered lines the odd numbered samples are removed.
To go from a progressive to an interlaced sequence, a progressive sequence is filtered in the vertical-time plane with a low pass filter having a diamond-shaped pass band and then quincunx subsampled to yield an interlaced sequence. A complementary sequence obtained with a high pass filter and also quincunx subsampled is known as a deinterlacing or helper sequence. If the interlaced and deinterlacing sequences can be recombined to perfectly reconstruct the original progressive sequence, then the same progressive sequence may be utilized for both a high resolution receiver (reconstructed version) and a lower resolution receiver (interlaced sequence by itself). Previous attempts to develop filter banks for this application (see, M. Isnardi, J. S. Fuhrer, T. R. Smith, J. L. Koslov, B. J. Roeder and W. F. Wedam, "Encoding for Compatibility and Recoverability in the ACTSystem," IEEE Transactions on Broadcasting, Vol. BC-33, No. 4, pp. 116-123, Dec. 1987; M. Tsinberg, "ENTSC Two-Channel Compatible HDTV System," IEEE Trans. on Consumer Electronics, Vol. CE-33, No. 3, pp. 146-153, Aug. 1987) have proven to be unsatisfactory because of the inability to perfectly recover the progressive sequence from the interlaced and deinterlacing sequences.
The design of perfect reconstruction filter banks has been extensively considered for the one dimensional case (see, P. P. Vaidyanathan, "Quadrature Mirror Filter Banks, M-band Extensions and Perfect Reconstruction Technique," IEEE ASSP Magazine, Vol. 4, No. 3, pp. 4-20, Jul. 1987; P. P. Vaidyanathan and Z. Doganata, "The Role of Lossless Systems in Modern Digital Signal Processing: A Tutorial," IEEE Transactions on Education, Vol. 32, No. 3, pp. 181-197, Aug. 1989; M. Vetterli and D. LeGall, "Perfect Reconstruction FIR Filter Banks: Some Properties and Factorizations," IEEE Trans. on ASSP, Vol. 37, No. 7, Ju. 1989, pp. 1057-1071). In two dimensions, some initial designs have been produced (see, E. H. Adelson and E. Simoncelli, "Orthogonal Pyramid Transforms for Image Coding," Proc. of SPIE on Visual Communications and Signal Processing, pp. 50-58, 1987; R. Ansari, "Two Dimensional IIR Filters for Exact Reconstruction in Tree-structured Subband Decomposition," Electronics Letters, Vol. 23, No. 12, Jun. 1987, pp. 633-634; G. Karlsson and M. Vetterli, "Theory of Two-dimensional Multirate Filter Banks," to appear in IEEE Trans. on ASSP, Jun. 1990; E. Viscito and J. Allebach, "The Analysis and Design of Multidimensional FIR perfect Reconstruction Filter Banks for Arbitrary Sampling Lattices," submitted for publication). However, these filters either do not perform satisfactorily (i.e. the reconstruction is not sufficiently perfect) or are not practical from an implementation standpoint. In particular, some of the prior art filters do not have finite precision filter coefficients. Other prior art filters are non-causal, i.e., they are infinite impulse response filters and thus are not suitable for use in the vertical-time plane.
Accordingly, it is an object of the present invention to provide a system, including easily implementable filter banks which have small size and finite precision filter coefficients, for converting a progressive sequence into an interlaced sequence and a deinterlacing sequence and for perfectly reconstructing the progressive sequence from the interlaced and deinterlaced sequences, so as to enable a single high definition television signal to be utilized by both a low definition interlaced receiver and a high definition progressive receiver.